1. Field of the Invention
The present invention relates to a balun transformer that converts between a balanced signal and an unbalanced signal.
2. Description of the Related Art
A balun transformer is a circuit that converts between a balanced signal and an unbalanced signal. A balun transformer is formed by integrating various components together by using a multilayer substrate, and is implemented in a communication apparatus such as a cellular phone. As balun transformers according to related art, for example, there are balun transformers as illustrated in FIGS. 11 and 12.
FIG. 11 is a circuit diagram illustrating a balun transformer 1F according to related art (see Japanese Unexamined Patent Application Publication No. 2005-166702). The balun transformer 1F includes an unbalanced terminal P1, balanced terminals P2 and P3, a low pass filter 21F, and a high pass filter 22F. The low pass filter 21F is connected between the unbalanced terminal P1 and the balanced terminal P2. The high pass filter 22F is connected between the unbalanced terminal P1 and the balanced terminal P3.
In a case where an unbalanced signal is inputted to the unbalanced terminal P1, the signal passes through the low pass filter 21F and the high pass filter 22F. When the signal passes through the low pass filter 21F, the phase of the signal is delayed by about 90 degrees. When the signal passes through the high pass filter 22F, the phase of the signal advances by about degrees. Consequently, the signals outputted from the balanced terminals P2 and P3 are balanced signals that differ in phase from each other by about 180 degrees. Likewise, in a case where balanced signals are inputted to the balanced terminals P2 and P3, a signal outputted from the unbalanced terminal P1 is an unbalanced signal.
FIG. 12 is a circuit diagram illustrating a balun transformer 1M (Merchant type) according to related art (see Japanese Unexamined Patent Application Publication No. 2003-110314). The balun transformer 1M includes an unbalanced terminal P1, balanced terminals P2 and P3, and ¼-wave length transmission lines 11M, 12M, 13M, and 14M. One terminal 111M of the transmission line 11M is connected to the unbalanced terminal P1. The other terminal 112M of the transmission line 11M is connected to one terminal 121M of the transmission line 12M. The transmission line 13M is located adjacent to the transmission line 11M, and is electromagnetically coupled to the transmission line 11M. The transmission line 14M is located adjacent to the transmission line 12M, and is electromagnetically coupled to the transmission line 12M. One terminal 132M of the transmission line 13M is connected to the balanced terminal P2. One terminal 141M of the transmission line 14M is connected to the balanced terminal P3. The other terminal 131M of the transmission line 13M and the other terminal 142M of the transmission line 14M are connected to ground.
A signal inputted to the unbalanced terminal P1 is transmitted through the transmission lines 11M and 12M. Then, the input signal is transmitted through the transmission line 13M by way of the electromagnetic coupling between the transmission lines 11M and 13M. The input signal is also transmitted through the transmission line 14M by way of the electromagnetic coupling between the transmission lines 12M and 14M. Signals outputted from the balanced terminals P2 and P3 are balanced signals that differ in phase from each other by about 180 degrees. In a case where balanced signals are inputted to the balanced terminals P2 and P3, a signal outputted from the unbalanced terminal P1 is an unbalanced signal.
In the case of the balun transformer 1F illustrated in FIG. 11, the phase of the signal is changed by means of the low pass filter 21F and the high pass filter 22F. FIG. 13 illustrates the transmission characteristics of the low pass filter and high pass filter. The vertical axis represents attenuation, and the horizontal axis represents frequency. A frequency f1 indicates the center frequency (operating point) of the frequency band being used.
The low pass filter 21F is designed so that the phase of the signal at the operating point is delayed by about 90 degrees. Moreover, as illustrated in FIG. 13, in the low pass filter 21F, the magnitude of the slope (hereinafter, the magnitude of the slope will be simply referred to as slope) of the attenuation characteristic becomes large at the operating point. That is, in the balun transformer 1F, when the low pass filter 21F is designed so that the phase of a signal that has passed through the low pass filter 21F is delayed by about 90 degrees, the slope of the attenuation characteristic of the low pass filter 21F becomes large.
The amplitude of a signal that has passed through a low pass filter is determined by the attenuation characteristic of the low pass filter at the frequency of the signal. Consequently, in the balun transformer 1F, when the frequency of a signal fluctuates within the frequency band being used, the amount of change in the amplitude of the signal that has passed through the low pass filter 21F becomes large.
The high pass filter 22F is designed so that the phase of the signal at the operating point advances by about 90 degrees. Moreover, as illustrated in FIG. 13, the slope of the attenuation characteristic at the operation point is large also in the high pass filter 22F. Consequently, in the balun transformer 1F, when the frequency of a signal fluctuates within the frequency band being used, the amount of change in the amplitude of the signal that has passed through the high pass filter 22F becomes large.
Therefore, when the frequency of a signal fluctuates within the frequency band being used, a large difference appears between the amplitudes of two balanced signals in the balun transformer 1F.
In the case of the balun transformer 1M illustrated in FIG. 12, the side of the circuit having the unbalanced terminal P1 and the side of the circuit having the balanced terminals P2 and P3 are electromagnetically coupled to each other. Therefore, the insertion loss is large in comparison to a case where the unbalanced terminal P1 and each of the balanced terminals P2 and P3 are directly connected by a wire.